FAST and IMAGE-FUV observations of a substorm onset S.B. Mende, C.W. Carlson, H.U. Frey, L.M. Peticolas, and N. Ostgaard On 6 February 2002 the NASA FAST satellite transited a substorm break up arc approximately 1 min after substorm onset as identified by the NASA IMAGE satellite far ultraviolet (FUV) instrument complement. These IMAGE data show that both the intense electron and proton precipitation features seen by FAST were not present prior to substorm onset. The most intense onset poleward surge was produced by superthermal electrons, and their energy spectrum and field-aligned angular distribution were consistent with wave accelerated electrons. The low energy ion fluxes in the E X B direction and magnetometer measurements confirmed the presence of waves in this feature. Thus the leading edge of the auroral surge was not produced by quasi-static field-aligned currents and related "inverted V" electric fields. The onset arc was also the poleward boundary of intense energetic protons. The FUV proton auroral images indicate that substantial part of the ion energy was carried by protons of energy >25 keV. Equatorward of the surge, there was a broader region of electron precipitation with embedded quasi-static "inverted V" electric fields. The FAST particle and magnetometer measurements are consistent with a field-aligned current structure in which the bulk of the upward current was carried by the "inverted VÕ" precipitation region. The region of the superthermal electrons carried very little net current. The high-density upward field-aligned current carried by the superthermal electrons was presumably balanced by oppositely directed downward currents carried by cold electrons. The ground onset location was separated from the closed/open field line boundary by an extended region of closed field lines. In this region there was some weak plasma sheet electron and proton precipitation, but there was no clear signature of any distant, prebreakup substorm onset activity. _______________ Journal of Geophysical Research, 108, A9, 1344, doi:10.1029/2002JA009787, 2003.